Posts (238+)

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September 2026

JUnit 5:Assert Behavior Before You Slice Spring

Prove an Order pricing rule with plain JUnit 5 — assertions, parameterized tests, and lifecycle — before you reach for a Spring test slice.

Java

`java.nio.file`:Paths, Copies, and Walks Without the Old IO Soup

Use Path and Files to read, copy, and walk directories in the JVM — without new File soup and without turning this into a Linux CLI tutorial.

Spring Boot

Spring Cache:Stop Querying for the Same Order Twice

Cache order reads with Spring @Cacheable and in-process Caffeine, evict on update, and keep JPA entities out of the cache.

Spring Boot

Flyway Without the `ddl-auto` Roulette

Version the orders schema with Flyway SQL migrations, turn off Hibernate ddl-auto outside local experiments, and prove the script with Testcontainers.

Spring Boot

When `@Valid` Fails, the Body Should Help:Problem Details and `@ControllerAdvice`

Turn Bean Validation failures and domain exceptions into RFC 7807 ProblemDetail responses with @ControllerAdvice so API clients get a body they can actually use.

Spring Boot

Profiles and Secrets:Stop Baking Production Into the Jar

Split Spring Boot config by profile, bind settings to a @ConfigurationProperties record, and inject secrets from the environment instead of committing them in the jar.

Spring Boot

RestClient After RestTemplate:One Client, Bounded Waits, Errors You Handle

Replace ad-hoc HTTP calls with Spring Boot 3 RestClient: inject the managed Builder, set timeouts, map JSON to records, and handle status codes without RestTemplate ceremony.

Algorithms

LIS:Longest Increasing Subsequence in n log n

Longest increasing subsequence: the O(n²) best-ending-here table, then patience / tails plus binary search for n log n.

Design Patterns

Decorator:Add Behavior Without Subclassing the Core

Replace AuditedRetryingCachedGateway subclasses with PaymentGateway wrappers you compose at the edge — logging, retry, cache — without a new type per combination.

Algorithms

LCS:Longest Shared Subsequence Without Requiring Contiguous

Find the longest common subsequence of two strings with a DP grid: match or skip, then reconstruct one shared sequence.